OVA OF MONOTREMES discovery rests with Owen and with Professor Poult on, who pointed out in 1884,1 that the ovum of OrnitTiorTiyncJius is very lareje as compared with those of other Mammalia (6 mm. as against *2 mm..)* that it is filled with yolk, and that it completely fills the follicle, being surrounded by two layers of follicular cells only. This latter fact was proved by CaldwelL Subse- quently Gyldberg 2 and I 3 described the ovarian ovum of Echidna, FIG. 46.—Ovarian egg of MchicLna. b, Basilar membrane ; fet follicular epithelium j o, oil globules ; vm, vitelline membrane ; y1, y2, yolk. (Partly after Caldwoll.) showing it to be identical with that of Ornitfoorfiynchus* ]Later still a more elaborate and beautifully illustrated £>aper was pub- lished by Caldwell4 upon the early stages of development in the Monotremata and Marsupials, in which the ovum of the former was accurately described (see Fig. 46). In the particulars mentioned above, the ovum of the Monotremata is practically identical with that of the large-yolked ova of the Sanropsida. 1 Quart. Jowm. Micr. Set, xxiv. 1884, p. 9. 2 Sf.S. Jen. Gesells* 188S, p. 1. 8 Proc. Moy. Phys. Soc. j&din. Tiii. 1885, p. 854, ' * PMfc. Trtms. clxxviii. 1887, p. 463.